This application is based on application No. 66169 filed in Japan on Mar. 10, 2000 and application No. 52005 filed in Japan on Feb. 27, 2001, the contents of which are incorporated hereinto by reference.
This invention relates to a safety vent apparatus of a sealed alkaline storage battery.
Sealed alkaline storage batteries such as nickel cadmium storage batteries and nickel hydride storage batteries are rechargeable batteries which use alkaline aqueous solutions as electrolyte.
As shown in FIG. 1, an alkaline storage battery 10 is a cylindrical metal external battery case 30 with a closed bottom end containing an electrode unit 12 together with electrolyte L and sealed at the open end of the external case 30 with a closing lid 80.
As shown in FIG. 1, the electrode unit 12 is formed by rolling a positive electrode plate 14, a negative electrode plate 16, and a gas permeable separator 18 intervening between the two electrode plates, into a spiral shape. The positive electrode plate 14 and the negative electrode plate 16 are formed by applying each of the active materials, nickel oxide and cadmium compounds to conducting electrode plates.
A positive electrode collector 20 is electrically connected to the upper end of the positive electrode plate 14, and the closing lid 80 is disposed above, and electrically connected to, the positive electrode collector 20.
The closing lid 80 is provided with a sealing plate 40 which is electrically connected to the positive electrode collector 20 and has a gas release hole 42 opened at its center, a safety vent unit 60 to close the gas release hole, and a positive electrode cap 70 to hold the safety vent unit 60 in a compressed state between the cap 70 and the sealing plate 40. The sealing plate 40 is caulked into the open end of the battery case 30 via an insulating gasket 76 to tightly close off and seal the external battery case 30.
In the sealed alkaline storage battery 10, when gas is generated internally as a result of conditions such as over-charge or over-discharge and the battery""s internal pressure exceeds a given pressure, the safety vent unit 60, is activated and gas and electrolyte inside the battery are vented to the outside to prevent battery case deformation, etc.
In general, gas generated inside a battery due to over-charging is consumed inside the battery. However, if charging is continued unchecked, the rate of gas generation can exceed the rate of gas consumption resulting in internal pressure rising above equilibrium values. Therefore, when internal battery pressure rises abnormally, battery case deformation and closing lid protrusion are prevented by venting gas via the safety vent unit described above.
The safety vent unit 60 is made up of a rubber ventage plate 62 and a spring 64 to power the ventage plate 62. In this safety vent unit 60, when internal battery pressure exceeds a given value, the spring 64 is compressed, the ventage plate 62 moves to open the gas release hole 42, and gas and electrolyte are ejected to the outside through the gas release hole 42 in the sealing plate 40 and gas venting holes 78 provided in the positive electrode cap 70.
Here, the activation pressure to begin operation of the safety vent unit is the pressure which begins to distort the spring in that unit. FIG. 2 shows the relation of the change in spring length to the amount of pressure applied to the safety vent unit (internal battery pressure). As shown by line a in FIG. 2, when activation pressure is set low, gas generated inside the battery can be vented at an early stage of internal pressure increase. This allows the previously mentioned dangers of battery case deformation and closing lid protrusion to be avoided at an early stage. However, since the gas generated inside a battery is due to electrolysis of water in the electrolyte, the quantity of electrolyte decreases as gas is vented. As a result, this can bring about loss in battery capacity after the safety vent has returned to its normal position. In contrast, when activation pressure is set high as shown by line b in FIG. 2, gas is not ejected even though the internal battery pressure is increased. This can invite the dangers of battery case deformation and closing lid protrusion.
Consequently, activation pressure is set to a suitable value which does not bring about loss in battery capacity and avoids the dangers associated with high internal battery pressure.
However, even when activation pressure is set to a suitable value, the rate of gas generation can be high during malfunctions such as extremely large charging current flow through the battery. In this case, even if the safety vent unit activates, if spring compression is small, the rate of gas ejection by the safety vent unit cannot approach the rate of gas generation.
The object of the present invention to provide a sealed alkaline storage battery which can eject large quantities of material such as gas and electrolyte, when internal pressure exceeds a given value, by setting the spring compressive load to length change (deformation) ratio to an optimum value.
To resolve the issues described above, the sealed alkaline storage battery of the present invention is provided with a sealing lid. The sealing lid is made up of a sealing plate electrically connected to an electrode unit, having a positive electrode plate, a negative electrode plate, and an intervening separator, which is contained in an external battery case having a cylindrical shape and a bottom; a safety vent unit which closes a gas release hole established through the sealing plate; and a positive electrode cap which accommodates the safety vent unit in a compressed state between the sealing plate and the cap. The safety vent unit is provided with a ventage plate to close the gas release hole and a spring to spring-load the ventage plate against the gas release hole. The ratio of compressive load per length change (deformation) of the spring is 50 N/mm or lower.
Since the ratio of compressive load per length change (compressive load/spring deformation) of the safety vent unit spring of the sealed alkaline storage battery described above is xe2x89xa650 N/mm, the distance the ventage plate can move when internal battery pressure exceeds a given value, can be large. Therefore, large quantities of material such as gas and electrolyte can be ejected outside the battery by widely opening the ventage plate from the gas release hole. As a result, external battery case deformation or rupture, due to a rise in internal battery pressure, can be prevented.